Reference liquids are used in many clinical analyzers to conduct potentiometric tests using twin ion-selective electrodes. One of those electrodes is contacted with the patient sample having an ion of unknown concentration, and the other electrode is contacted with the reference liquid having a known concentration of ions. Most preferably, such ions are highly concentrated to the point of near saturation, for the reason that the junction potential between the reference liquid and the sample needs to be dominated by the reference liquid concentration, which means the latter must be highly concentrated. The reference liquid is stored in a reservoir that has to be replenished. See, e.g., U.S. Pat. No. 4,740,274 for further details on the reference liquid.
For years the most common reservoir for such a reference liquid has been a glass or plastic vial that is capped with a rubber seal that is machine-opened and closed. Before the vial is replenished, it has to be removed and cleaned to maintain an accurate concentration of the ions. Such a procedure has been a problem because it is time-consuming, labor intensive, and if not done properly, introduces errors due to the concentration of the liquid being altered.
There has therefore been a long-standing need for a disposable, single-use reservoir which, although preventing evaporation, readily allows access to the liquid by an aspirator. "Single-use" as used here means used until the liquid contents of the reservoir have been exhausted without replenishment.
The most obvious solution to the need was to form the reservoir as a simple plastic body with an aperture at the top sized to seal on the aspirator when it is inserted. In that fashion, the aspirator acts as a stopper when it is kept in the reservoir aperture when no reference liquid is being dispensed in the analyzer. However, it was soon discovered that this design was unsatisfactory because insertion of the aspirator into its sealing position created severe pumping action in the reservoir due to flexing of the top of the reservoir that interfered with pressure sensing that is otherwise necessary with such aspirators.
The next step was to enlarge slightly the aperture for the aspirator, so that a complete seal, and hence pumping, did not occur due to the "leak" created. The aperture could not be made too large, however, as evaporation then is too substantial. However, this was found to be a failure in that the "leak" portion of the aperture created a sufficient capillary path for liquid when it sloshed, as to cause unacceptable crusting of the high salts contents of the liquid at that "leak" portion.
Next, a flat cover, apertured for the aspirator, was designed to fit over and close off the top of the reservoir. However, this sealed to the top of the reservoir along a flat surface. The flat cover attempted to achieve controlled venting by a long diffusion path between the cover and reservoir. However, economical manufacturing tolerances would not allow the gap to be controlled tightly enough to prevent excessive evaporation in worst case conditions.
Thus, for many months, attempts have been made to create an acceptable disposable alternative for the permanent glass reference liquid reservoir heretofore used, all without success.